Modems are used to transmit data between data terminal equipment over a communication line. Typically, a modem is connected between a computer, such as a personal computer (PC) and a telephone or other communication line (e.g., an ISDN line or a coaxial cable), such that the computer can communicate with another computer, which also is connected to a modem, by way of the telephone line. The modem receives digital data from the PC and modulates or converts the digital data to analog data signals (in the case of a plain old telephone system (POTS) line) and transmits the analog signal over the telephone line to another modem, connected to another computer or PC. The modem also receives data, such as analog data, demodulates and converts the analog data to corresponding digital data, and provides the digital data to the computer. Of course, the modem must also perform many other functions, such as modulation, filtering, compression/decompression, and protocol establishment. Such modem devices are relatively well known and widely commercially available. For example, the Cardinal Technologies, Inc. MVP288IS modem provides 28,800 bps data transmission using industry standard protocols, data compression and error correction.
With the development of increasingly powerful PCs able to perform sophisticated data manipulation and display graphic and video images, along with the increase in accessibility to digital information, such as via the internet, computer users are demanding that communication lines and modems be able to handle data at increasingly faster rates. Accordingly, a protocol for transmitting digital data has been established, known as ISDN (Integrated Services Digital Network) which is described in the ANSI standard T1.601 entitled, "Integrated Services Digital Network (ISDN)--Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification), published 1992 by the American National Standards Institute. Digital data can be transmitted at faster rates than analog data and devices receiving such digital data are no longer required to convert the data from analog to digital, and vice-versa. Accordingly, modems which are capable of operating in accordance with the ISDN protocol are able to transmit, receive and process data more efficiently than modems which transmit and receive only analog data. Although many computer users do not presently have access to an ISDN communication line, ISDN communication lines are becoming more commonplace. Accordingly, it is desirable to provide a modem which is upgradeable to operate in accordance with the ISDN protocol.
The present invention comprises an intelligent cable assembly for connecting an ISDN upgradeable modem to an ISDN communication line. The ISDN upgradeable modem includes an incremental amount of hardware to support the ISDN protocol, a processor with a direct memory access (DMA), and an interface means for communicating with a host processor. Through the use of a modem processor which supports DMA, firmware supporting the ISDN protocol can be downloaded to the modem processor to allow the modem to operate in accordance with the ISDN protocol. The cable assembly includes a housing which contains the electronic components necessary for receiving a signal transmitted over the ISDN communication line and isolating and converting the ISDN signal to a synchronous or asynchronous serial or parallel digital signal. In contrast, prior art ISDN modems support the ISDN protocol using specialized hardware, such as a separate processor and a separate controller, which correspondingly raises the cost and complexity of the modem.
The cable assembly of the present invention allows a flexible and inexpensive means of upgrading a modem to communicate over an ISDN communication line.